The invention relates to an arrangement for the automatic critical focusing of optical systems in mirror reflex cameras provided with a partially transmitting reflex mirror, an image field lens arranged in the viewfinder ray path as well as a range-finder image plane that is equivalent to the film plane; photoconductive cells, which are arranged in the range-finder image plane or in the exit pupil of the optical system, emitting signals which, through an electric circuit, bring about an indication of sharpness in the viewfinder and/or the automatic setting of the taking lens.
Principles of the automatic focusing of camera lenses are in many cases based on projecting an image of the object to be critically focused and on obtaining the required sharpness signal in conjunction with measurement receivers, which are provided either in the image plane itself or in the exit pupil plane.
If the image of the object to be critically focused is projected by the taking lens itself, it is known on mirror reflex cameras to produce an image plane that is equivalent to the film plane by means of beam splitters and further auxiliary lenses and mirrors in the camera base. It is furthermore known to produce this range-finder image plane in the viewfinder ray path, that is to say to provide it downstream of the image field lens. The disadvantage of these arrangements is a considerable expenditure of optical components which, in some cases, additionally have to be provided in a movable manner. Another disadvantage is the large adjustment expenditure in order to set the range-finder image plane with sufficient accuracy.